Improve the credential icon

[Abestanis]

The current implementation of the credentials icon has a couple of issues:

• The text inside the input can overlap with the icon.

  The password field on the Amazon login page:

• The same applies for the hint text.

  The username field on the Nextcloud login page:

• Other UI elements can also overlay over the icon.

  The username field on Apples login page:

• We are potentially overwriting a background image of the input that the website specified.

• We are potentially restoring the wrong title if the website would change it while we hovered over the credentials icon.

To fix these problems, this pull requests uses a separate img element that is positioned above the input field, similar to the dropdown menu.

This fixes the problems described above,

but it requires us to move the credential icon manually, if the input moved.

This is a test web page in a zip that can be used to test the behavior of the credentials icon on moving inputs.